Conventional bobbin-wound transformers are used in many electronic devices. Bobbin-wound transformers, which are generally formed by winding conductive wires having insulating layers about a bobbin, are simple in construction and have adequate performance for many applications. However, bobbin-wound transformers have several limitations. Several of these limitations result from the difficulty in removing heat from the transformers. Insulating layers that cover the winding wires hinder conduction of heat from the wires, while the windings interfere with air flow to inner layers of the windings and thus decrease convective heat transfer. As a result of problems with cooling bobbin-wound transformers, there are electrical conversion and material use inefficiencies that either limit the use or operation of these transformers, limit the power density, or require more space or additional resources to provide adequate cooling. Toroidal transformers have been developed to address the problems with bobbin-wound transformers, but these too have problems.
More specifically, it is well known in high frequency switching power supply applications to use the popular geometry of ferrite cores, e.g., EE, EI, PQ, ETD, EC, RM, and similar type of cores, in conjunction with the use of an insulating bobbin to position the windings. However, the resulting transformers have serious problems in modern high density switching power supply applications. Such transformers are bulky and are difficult to cool. Usually the innermost winding is buried under several layers of insulation and thus suffers the most from the latter disadvantage, i.e., the heat transfer mechanism of such a construction is through all of the other upper windings and insulation layers. This type of transformer has extremely high thermal resistance to ambient and needs to use over-sized copper wiring to meet hot spot temperature limits. Its performance improves only marginally by impregnating the transformer with varnish or some other filler.
The use of toroidal transformers is an effective solution to answer power density and thermal issues. However, the biggest problem in prior art toroidal transformers is the high potential safety insulation between the primary and the secondary low voltage windings. For example, U.S. Pat. Nos. 4,551,700, 5,838,220, and 6,300,857 each suggest methods to meet these safety insulation needs. These prior art methods still seriously affect the manufacturing yield in high volume applications. Applying insulation layers over the primary winding using an insulation tape or film is too cumbersome while using a sleeve on one of the windings is still time consuming.
A toroidal transformer constructed using techniques suggested in above-mentioned prior art still also has thermal limitations. Inherently, most of the windings of the toroidal transformer are exposed to ambient air depending upon the insulation method. An insulating cap or a sleeve on the winding increases its thermal resistance to ambient. Using triple insulated wires is not a viable option due to the difficulty faced in winding the wires on toroids because of the spring-back effect that occurs during winding.
What is needed is a transformer having improved thermal performance. What is also needed is a transformer having improved power density. It is also desirable to have a transformer with a smaller footprint than conventional bobbin-wound transformers. In addition, it is desirable for the transformer to use less material than conventional bobbin-wound transformers.